1. Field of the Invention
This invention relates to communication systems, and more particulary, to a method and apparatus to detect tones in a noisy signal environment using noise detection and dynamic thresholding.
2. Description of the Related Art
Communication systems often utilize a loop to connect terminal equipment such as telephone instruments, facsimile machines, private branch exchanges, key telephone systems, voice mail systems, modems, computers, alarm systems, radio controlled systems and telephone answering machines as well as many other devices. These devices are often referred to as Customer Premises Equipment (“CPE”). To perform various signaling applications such as telephone dialing, data entry, caller ID, and storage system control, Dual-Tone Multiple Frequency (“DTMF”) signals are often used. A DTMF signal is defined by two superimposed sinusoidal waveforms with frequencies generally chosen within the voice band.
Detection of multiple-tone, multiple frequency (MTMF) signals, such as Dual-Tone Multiple Frequency (DTMF) signals, in environments where one or more noise signals is present is generally recognized by those of ordinary skill in the art to be a complex issue. Detection of MTMF signals is particularly difficult when the noise signals share the voiceband used for inband MTMF signaling. One source of noise interference which impedes MTMF detection is referred to as “talk-off”. Talk-off occurs whenever a signal tone detector erroneously accepts signal imitations, such as those produced by speech or music, as valid signals. These noise signals can imitate some of the temporal and spectral characteristics of signaling tones. These imitations are likely to trigger, or talk-off, signal tone detectors. An important goal in designing such detectors is making them immune to these signal imitations.
An additional problem with signal tone recognition is that signaling tones must, in some situations, compete with speech, music or other extraneous background noise. The existence of these complex noise signals introduces spectral components into the signal to be recognised that distort and ultimately impair the detection of valid signaling tones. A signal detector is said to have been “talked down” whenever it fails to recognize valid signaling tones that were masked by noise signals such as speech, music or other noise.